Answers: run
http://www.windirstat.info
it will tell you wnats taking up space on your hard disk.
honestly, firefox and AIM dont take up much space and using something smaller will compromise on features, so they are worth keeping as is. You could run opera or Gaim as possible alternatives, but they wont save you much space.
run windirstat, go over your hard disk, and remove any unnecesary files.
If you use a pop3 mail reader (outlook, thunderbird, etc), you may find deleting archived emails with large file attachments will also save you a lot of room.